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Subject: Re: [virtio-dev] Re: [PATCH v2 1/3] shared memory: Define shared memory regions
* Cornelia Huck (cohuck@redhat.com) wrote: > On Fri, 22 Feb 2019 11:54:31 +0000 > "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote: > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > Define the requirements and idea behind shared memory regions. > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > --- > > content.tex | 2 ++ > > shared-mem.tex | 33 +++++++++++++++++++++++++++++++++ > > 2 files changed, 35 insertions(+) > > create mode 100644 shared-mem.tex > > > > diff --git a/content.tex b/content.tex > > index 836ee52..3dd504c 100644 > > --- a/content.tex > > +++ b/content.tex > > @@ -371,6 +371,8 @@ making any more buffers available. When VIRTIO_F_NOTIFICATION_DATA > > has been negotiated, these notifications would then have > > identical \field{next_off} and \field{next_wrap} values. > > > > +\input{shared-mem.tex} > > + > > \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation} > > > > We start with an overview of device initialization, then expand on the > > diff --git a/shared-mem.tex b/shared-mem.tex > > new file mode 100644 > > index 0000000..85b0c55 > > --- /dev/null > > +++ b/shared-mem.tex > > @@ -0,0 +1,33 @@ > > +\section{Shared Memory Regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions} > > + > > +Shared memory regions are an additional facility > > +available to devices that need a region of memory that's > > +continuously shared between the host and the guest, rather > > +than passed between them in the way virtqueue elements are. > > + > > +Example uses include shared caches and version pools for versioned > > +data structures. > > + > > +The region is chosen by the host and presented to the guest, as > > +such it is useful in situations where the memory is accessed on > > +the host by other libraries that can't safely access guest RAM. > > This explanation looks good to me. > > > + > > +Shared memory regions MUST NOT be used to control the operation > > +of the device, nor to stream data; those should still be performed > > +using virtqueues. > > The 'MUST NOT' makes it look like a normative statement; however, this > is more like design advice? Hmm I'm not sure - the intention of this line is to try and stop people using it as a hack to avoid standardising protocol for no good reason. > > + > > +A device may have multiple shared memory regions associated with > > +it. Each region has a \field{shmid} to identify it, the meaning > > +of which is device-specific. > > + > > +Enumeration and location of shared memory regions is performed > > +using a transport-specific data structure and mechanism. > > + > > +Memory consistency rules vary depending on the region and the > > +device. Devices MUST define the required behaviour for each > > +region. > > Same here. OK, so should I reword this? > > + > > +The guest physical address and the host virtual address MUST NOT > > +be used to identify structures within the memory regions; all > > +addressing MUST be relative to the start of a particular region. > > + > > For that high-level overview, I'm not sure if any normative statements > are needed/wanted, or whether those should be confined to the individual > transport or device type definitions... I think this one is stronger than the previous two; if people start passing GPA/HVA in the underlying structures things are going to get messy. Dave > The text on its own looks good to me. > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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